What we notice, standing at the door: apart from the obvious, male, female, old or young, we pay attention to their language (check if they hold a magazine or newspaper), citizenship (in Japan I greet with Konnichiwa or Ohayogozaimasu), in Thailand, sawasdeekrap, and so on.. if I know it is in their language I try to welcome the guests and hope I guessed right.
Another thing: Do I smell something? Alcohol? Drugs (one can smell Marijuana)… If a guest coming aboard is drunk or intoxicated by any drug, it can happen that he or she is not allowed to enter the plane.
Does a guest come with a handicap? A disease? Does he/she look healthy…
There are probably more things we notice, in short everything out of the ordinary and we do this in matter of seconds.
The first impression is often the right one and we do refuse passengers who might be a danger for the safety of that flight.
So far I have refused four passengers and was luckily backed up by the purser and captain. All of these were drunken passengers. Three men and one woman, over a period of thirty years!